Although most ovarian cancer patients respond very well to first-line clinical therapy, the vast majority eventually succumb to recurrent, chemoresistant disease. This highlights the urgent need for studies that investigate mechanisms of relapse and identify potential therapeutic targets for the treatment of recurrent ovarian cancer Our lab previously identified ovarian cancer stem cells (OvCSC) marked by coexpression of ALDH and CD133, and discovered that these cells preferentially express high Nuclear Factor of Activated T-Cells 3 (NFAT3) levels. We have observed that cells with a constitutively active NFAT3 (cNFAT3) demonstrate dramatic growth reduction in vitro and significantly smaller tumor size in vivo. Furthermore, cNFAT3 cells are more resistant to cisplatin therapy than control cells; conversely, treatment with VIVIT, an NFAT inhibitor, sensitizes ovarian cancer cells to cisplatin. We hypothesize that NFAT3-induced cell cycle retardation decreases the sensitivity of ovarian cancer stem cells to chemotherapy, thereby allowing them to survive through treatment and later cause recurrent disease. We therefore propose to investigate mechanisms of quiescence and chemoresistance in OvCSC, with the ultimate aim of elucidating the role of OvCSC in relapse. Cell cycle analysis and quiescence-specific FACS analysis will be used to clarify the role of NFAT3 in growth retardation. Array analysis will be used to identify NFAT3-regulated cell cycle targets for further analysis and identification of potential therapeutic targets. We will usea novel single-cell microfluidics culture device that allows direct quantification of asymmetric vs. symmetric divisions to study the effects of NFAT3 on stem cell fate decisions. We will subsequently investigate the effects that NFAT3 has on chemoresistance and tumorigenesis in vitro and in vivo. We will also investigate quiescence-associated proteins and classic chemoresistance mechanisms, such as expression of P-glycoprotein, to identify the mechanism by which NFAT3 protects cells during cisplatin therapy. Finally, primary ovarian tumor samples will be studied to correlate NFAT3 levels with progression free survival, overall survival, and the presence of chemoresistant disease. Taken together, these studies of quiescence and chemoresistance will provide critical insight into mechanisms by which ovarian cancer cells and CSC are able to survive through cisplatin therapy, causing relapse.